This invention relates to FM stereophonic broadcasting systems of the type which utilizes compansion of the difference signal and, more particularly to a decoder for use in the receiver of such systems which employs stepped gain control circuitry.
U.S. Pat. No. 4,485,483, the disclosure of which is hereby incorporated herein by reference, describes a compatible stereophonic broadcasting system which utilizes companding of the difference signal. In this system stereophonically related audio frequency source signals L and R are matrixed to obtain stereophonic sum and difference signals M and S, respectively. At the transmitter, the difference signal is used to amplitude-modulate a first sub-carrier signal and is also compressed and the compressed version S' used to amplitude-modulate a second sub-carrier of the same frequency but in quadrature phase relationship with the first. Suppressed-carrier, double-sideband modulation is employed, with the frequency of the sub-carrier signal being sufficiently high to assure a frequency gap between the lower sidebands of the modulated sub-carrier signals and the sum signal. A conventional low-level phase reference pilot signal, lying within the gap, is provided for detection purposes at the receiver. The composite baseband signal comprising the sum signal, the two modulated quadrature sub-carrier signals, and the pilot signal (which composite signal will sometimes be referred to herein as an "FMX" signal) is frequency modulated onto a high frequency carrier for broadcast. ("FMX" is a registered trademark of Broadcast Technology Partners, Bloomfield Hills, Mich.)
The receiver of the system includes a demodulator for deriving the sum signal, the usual difference signal S and the compressed difference signal S', and an expander for complementarily expanding the derived compressed difference signal. The expanded noise-reduced version of the difference signal is matrixed with the received sum signal to obtain the original audio frequency source signals L and R. If desired, the usual difference signal S, instead of the noise-reduced version of the difference signal S', may be matrixed with the sum signal. Thus, the decoder described in U.S. Pat. No. 4,485,483 makes a one-time choice between either S' or S and thereafter only the initially selected version of the difference signal is matrixed with the sum signal.
Commonly assigned U.S. Pat. No. 4,602,381, the disclosure of which is also hereby incorported herein by reference, describes an adaptive expander which works on the sum of signals S and S', and uses the usual difference signal S as a reference signal for controlling the expansion of the compressed difference signal S' so as to cause the amplitude of the expanded difference signal to be equal to the level of the usual difference signal. The expanded noise-reduced version of the difference signal is matrixed with the received sum signal to obtain the origional audio frequency source signals L and R. This adaptive expander, which will sometimes be referred to hereinafter as "the conventional "FMX" expander", depends for its operation on a relatively expensive analog voltage controlled amplifier.
In view of the relatively large increment of cost attributable to the voltage controlled amplifier, there is a need for an expander capable of performing on the quadrature difference signal the complement of the function performed by the compressor at the transmitter without using a voltage controlled amplifier. Accordingly, a primary object of the present invention is to provide a decoder for use in the receiver of an "FMX" broadcasting system capable of providing acceptable complementary expansion of a received "FMX" signal which is less expensive to manufacture than those heretofore available.